Rice is one of the three major cereal crops and is a staple food for more than a third of the world's population. Drought is one of the major abiotic stress factors limiting crop productivity worldwide. Global climate changes may further exacerbate the drought situation in major crop-producing countries. Although irrigation may in theory solve the drought problem, it is usually not a viable option because of the cost associated with building and maintaining an effective irrigation system, as well as other issues, such as the general availability of water. Thus, alternative means for alleviating plant water stress are needed.
Upon exposure of plants to drought conditions, many stress-related genes are induced and their products are thought to function as cellular protectants of stress-induced damage. The expression of stress-related genes is largely regulated by specific transcription factors. Members of the AP2, bZIP, zinc finger, and MYB families have been shown to have regulatory roles in stress responses. The rice and Arabidopsis genomes code for more than 1300 transcriptional regulators, accounting for about 6% of the estimated total number of genes in both cases. About 45% of these transcription factors were reported to be from plant-specific families (Riechmann et al., (2000) Science 290: 2105-2110).
Plant modification for enhanced drought tolerance is mostly based on the manipulation of either transcription and/or signaling factors or genes that directly protect plant cells against water deficit. Despite much progress in the field, understanding the basic biochemical and molecular mechanisms for drought stress perception, transduction, response and tolerance remains a major challenge. Utilization of the knowledge on drought tolerance to generate plants that can endure under extreme water deficit condition is even a bigger challenge.
Rice is a staple food for greater than one third of the world's population and worldwide total rice planting area is approximately 1.5 million square kilometers. In 2009 global rice production was over 670 million tons, second only to maize.
Approximately 20% of rice growing areas worldwide are prone to drought. Drought is a particularly important issue for rice production. About 5000 liters of water are needed to produce one kilogram of rice, approximately double the needs of other crops.
Despite efforts to develop drought-tolerant rice plants, very few attempts have been shown to improve grain yields. Examples of positive effects include transgenic rice plants expressing SNAC1 (Hu et al., (2006) Proc Natl Acad Sci USA 103: 12987-12992) and OsLEA3 (Xiao at al., (2007) Theor Appl Genet 115: 35-46), which was shown to improve grain yield under field drought conditions.
Heterotrimeric G-proteins are key signal transduction components that couple the perception of an external signal by a G-protein coupled receptor (GPCR) to downstream effectors. More than one third of mammalian signaling pathways depend on heterotrimeric G-proteins, including vision, taste, olfaction, hormones, and neurotransmitters. G-protein coupled signaling pathways are targets of approximately half of all pharmaceuticals.
The G-protein complex is comprised of Gα, Gβ and Gγ monomeric subunits that assemble as a heterotrimer that physically associates with a GPCR. Activation of the GPCR triggers the Gα subunit to exchange GDP for GTP, thus activating the G-protein. Once active the heterotrimeric complex dissociates from the GPCR and the Gα subunit separates from the Gβγ heterodimer. Both GTP-bound Gα and the Gβγ heterodimer transduce the signal to downstream effectors.
Heterotrimeric G-proteins have been studied extensively in animals. To date, 23 Gα, 6 Gβ and 11 Gγ genes have been reported in mammals (Vanderbeld and Kelly (2000) Biochem. Cell Biol. 78: 537-550). The alpha subunits are classified into four subfamilies: Gs, Gi, Gq, and G12. In contrast, relatively little is known about the role G-proteins play in plants. Loss-of-function mutants in the Gα subunit of rice and Arabidopsis are completely viable, but show several characteristic developmental attributes. The rice mutant exhibits shortened internodes, rounded seeds, and partial insensitivity to gibberellin, whereas the Arabidopsis mutants have rounded leaves and altered sensitivity to a number of phytohormones (Ashikari et al. (1999) Proc. Natl. Acad. Sci. 96: 10284-10289; Fujisawa et al. (1999) Proc. Natl. Acad. Sci. 96:7575-7580; Ueguchi-Tanaka et al. (2000) Proc. Natl. Acad. Sci. 97: 11638-11643; Wang et al. (2001) Science 292: 2070-2072; (Ullah et al. (2001) Science 292: 2066-2069). A loss-of-function mutant in the Gβ subunit of Arabidopsis (AGB1) exhibits several defects including short, blunt fruits, rounded leaves, and shortened floral buds (Lease et al. (2001) Plant Cell 13: 2631-2641).
It can be seen that there is a continuing need to develop drought tolerance in plants, particularly rice.
It is an object of the present invention to modulate Gα proteins such as RGA1, in plants to engineer drought tolerance and increase seed yield under such conditions.
Other objects will become apparent from the description of the invention, which follows.